Due to the recent development of broadband, the demand for large-capacity high-speed radio communication is increasing. As the capacity of communication has increased, a third generation has become widespread for mobile phones and further, a fourth generation will develop in the future. Due to this, a wideband amplifier for transmission that is used in a based station of a mobile phone, an increase in output power and improvement of efficiency are further demanded.
The above-described wideband amplifier is also used in the transmission unit of a radar. For the radar's high power amplifier, an increase in output power, an increase in band, and improvement of efficiency are further demanded for extension of detection distance and improvement of resolution. If high efficiency is achieved, power that is consumed as heat is reduced, and therefore, the cooling unit may be more compact and it is possible to reduce the size and cost.
In order to increase output power, the transistor size is increased and further, an integrated circuit is formed by using a high power transistor capable of a large current operation on a high voltage, for example, a GaN HEMT.
Further, as a method for creating a design to widen the band, a distributed amplifier is known. The distributed amplifier has an input-side transmission line, an amplification circuit, and an output-side transmission line. On the input-side transmission line, input terminals in a plurality of amplification stages of the amplification circuit are sequentially connected periodically, and in the output-side transmission line also, output terminals in a plurality of amplification stages are sequentially connected periodically. Further, the opposite side of the input terminal of the input-side transmission line is terminated with a resistor and the opposite side of the output terminal of the output-side transmission line is terminated with a resistor.
The above-described distributed amplifier has an advantage that gain is constant across wide frequencies, but there is a problem in that efficiency (output power/power consumed as DC) is reduced because the terminating resistor absorbs power.
In order to solve the above-described problem, it has been proposed that the output-side terminator (terminating resistor) is removed and the width of the transmission line in each stage of the output-side transmission line is gradually increased in the above-described distributed amplifier. The line width corresponds to the magnitude of impedance. The signal that is amplified in the amplification stage is guided to the output-side transmission line, flows through the thick transmission line whose impedance is low, and is guided to the output terminal. This distributed amplifier does not use a terminator, and therefore, the power that is consumed as DC (direct current) is reduced, and as a result, high efficiency is achieved.
The output power of the distributed amplifier is in proportion to the output power of the unit amplifier in each amplification stage and the number of stages. Consequently, if the number of amplification stages is increased, it is possible to increase the output power of the distributed amplifier. However, as described previously, if the number of stages is increased, it is also necessary to increase the line width of the transmission line in each stage of the output-side transmission line, and in view of the layout, it is not possible to increase the number of stages so much (normally, about five to ten stages).
Further, the cutoff frequency of the distributed amplifier is considered. The cutoff frequency is the upper limit frequency at which the amplification factor is 1 or higher, corresponding to the upper limit frequency of the wideband amplifier. The cutoff frequency of the distributed amplifier is in inverse proportion to the product of the capacitance that is represented by an equivalent circuit of the amplification stage and the square root of the number of stages. Because of this, if the size of the transistor that is used in the amplification stage is increased for the purpose of increasing the output power, the cutoff frequency is reduced. Similarly, there is a problem in that the cutoff frequency is reduced even if the number of stages is increased.
On the other hand, it has been proposed to branch the circuit into two distributed amplifiers from the input-side transmission line and to combine their outputs. Due to this, it is possible to halve the size of the transistor that is used in the amplification stage although the total transistor size is the same, and in the case where the circuit is branched from the same portion of the input-side transmission line, the two amplification stages are connected, and therefore, the total capacitance is the same and it is not possible to increase the cutoff frequency.
Related-art techniques are disclosed in    Japanese Laid Open Patent Publication No. H08-078976,    Japanese Laid Open Patent Publication No. H06-013806,    Japanese Laid Open Patent Publication No. 2005-086634 and    Philippe Dennler, et al., “8-42 GHz GaN Non-Uniform Distributed Power Amplifier MMICs in Microstrip Technology”, IMS2012.